We’re a long way from beating cancer, but scientists are gaining ground. For the past few weeks, we’ve been highlighting recent steps forward. This week: cancer-fighting viruses.

“The same genes that cancer cells exploit to their advantage—the ones involved in the growth, life, and death of your tissues—are involved in fighting viruses,” explains John Bell, a senior scientist at the Ottawa Hospital Research Institute and a professor of medicine at the University of Ottawa. “A tumour becomes a tumour by getting rid of some of the genes that would control its growth—but it also throws away genes that would control its ability to fight virus infection. That makes tumours exquisitely sensitive to virus infection, while your normal tissues can resist it.”

Consequently, Bell and other researchers around the world have worked on engineering viruses that will home in on cancer cells. “We can engineer a virus so that the only thing it can infect and be a parasite of is a cancer cell,” he says. “We’ve engineered viruses that can infect and destroy only cancer cells, leaving normal tissues unscathed, so there won’t be all the side effects.”

That’s not all. “Not only does the virus infect the cancer cell and destroy it, but it alerts the immune system,” Bell continues. Alerting the immune system to the virus also awakens it to the presence of the cancer, prompting the development of an antitumour response. “The first virus we worked on eight or nine years ago is now in the final stages of clinical testing for liver cancer, and a second virus we’ve been developing is now in two different trials in Canada—in lung, breast, and esophageal cancers—with two more planned by the end of 2017, focusing on prostate and cervical cancers.”

Even once the virus has roused the immune system, the tumour may still be able to switch it off again. To address this problem, Bell and other scientists are beginning to explore using viruses in combination with medications that can block that “off” button for a time: the former turns the key in the ignition, while the latter presses down on the gas pedal.

“That’s really what our lung cancer trial is: to have the virus go in and initiate an immune response, and then to add a drug called pembrolizumab that makes the immune system super-active,” Bell says. “Over the years, we’ve learned that the only way to treat cancer is to use combination therapies, because cancer is too good at evading single modalities.”